Pulsar master branch - 2.8.0 is on JDK11 !

[Enrico Olivelli <eo...@gmail.com>]

Hi all,
I just wanted to note that we merged the patch that switched CI and
the docker images to JDK11.

Things you have to know:
- you can still use JDK8 and JDK11 in order to develop Pulsar
- source and target releases are JDK8, so we cannot use language
features or libraries that are not available on JDK8
- CI is running on JDK11 for PR validation
- Pulsar Server and Pulsar Client still run well on JDK8 - no breaking
change for users
- Official Docker images from 2.8.0 onwards are based on OpenJDK11

Many orgs have been running Pulsar on JDK11 for long time, so we do
not expect surprises for Docker users.

Thank you very much to everyone who participated in this work,
especially to Lari who completed the work I started a couple of months
ago and made it possible to have a full working CI environment.

Cheers
Enrico